The present invention relates to a broad beam electron gun and method and more particularly to an extended spark cold cathode electron gun suitable for radiation processing and for laser stimulation, operable either in pulsed or DC mode.
The high energy electron beam obtainable from an electron gun is capable of inducing a rearrangement of molecules in many materials. This has been used for synthesizing and cross-linking of polymers, hydrogenation, the curing of paint and a number of similar processes and operations. Reference is made to U.S. Pat. No. 3,925,670 for "Electron Beam Irradiation of Materials Using Rapidly Pulsed Cold Cathodes", assigned to the present assignee, which gives an overview of the present state of the art.
Currently available electron guns can be roughly classified in terms of electron emission methods; that is, into thermionic (hot) cathode electron guns with continuous (CW) operation, pulsed cold cathode electron guns and field emission electron guns. The hot cathode guns may be compared to conventional triodes in which electrons are ejected from a hot filament. In the cold cathode operation a pulsed negative potential applied to a metallic projecting element is employed to cause generation of a plasma from which electrons are drawn. In the field emission guns a high field is used to draw electrons from a fine tungsten point. None of these types of electron guns is capable of providing all the characteristics and requirements desired particuarly with respect to efficiency, reliability, and ruggedness.
The usefulness of a thermionic cathode is offset by numerous drawbacks in structural and operational parameters, while the superior features of a cold cathode are limited by its inability to produce pulses of a long duration. If the projection of a cold cathode is shaped as a longitudinal edge, a plasma sheet from which the electrons are to be extracted forms in front of the cathode. The sheet is inherently unstable and not easily controlled. Within a very short period, for example, several microseconds, it diffuses across the cathode-anode gap and produces an arc which effectively interrupts propagation of the beam. The pulses obtainable from a cold cathode electron gun are thus of very short length and of limited usefulness for certain processes. Polymerization, for example, is affected not only by the radiation dose, that is, the total amount of radiation directed onto the material, but also by the rate at which the dose is delivered. The high dose rate of very short electron beam pulses, of the order of microseconds, elicits chemical reactions which may be different from those produced by the impact of long pulses or continuous radiation.
In choosing between a thermionic and a cold cathode electron gun for a specific application it has thus been necessary to make a compromise decision and to weigh the respective advantageous characteristics of one against the other. There is, therefore, a need for a new and improved electron gun for radiation processing, laser stimulations and other purposes.